Electrowetting-based control of wetting transition of a nanodroplet on pillar-arrayed surfaces

Two extreme wetting states, a highly non-wetting Cassie state and a wetting Wenzel state, can coexist or even mutually convert on patterned surfaces. Such a conversion process may be spontaneous or induced by external stimuli. This work studies the wetting transition of a nanodroplet on pillar-arrayed surfaces induced by an external electric field via an energy-minimization method in conjunction with molecular dynamics (MD) simulations. The simulation results reveal that driven by the electric field, the initial Cassie state could go through a partial wetting state, and eventually converts to the Wenzel state. The free-energy landscape reveals that there are multiple local energy minima, corresponding to multiple metastable states. For the metastable Cassie state, the wetting transition is irreversible, i.e., the droplet would remain in the Wenzel state when the electric field is removed. Conversely, the spontaneous dewetting transition from the Wenzel to the Cassie state can occur, if only the Cassie droplet is in a global energy minimum configuration. Thus, the stable Cassie wetting configuration is essential for triggering the spontaneous dewetting transition.